In recent years, one such function which is installed in a storage apparatus that has been attracting attention is a virtualization function known as thin provisioning. A thin provisioning function is a function which provides a host with a virtual logical volume (referred to hereinbelow as a virtual volume), and which, when a request to write data to the virtual volume is supplied from the host, dynamically assigns a storage area to the virtual volume. The thin provisioning function is advantageous in providing to the host a virtual volume with a larger capacity than the storage area that is actually available and in affording the construction of a low-cost computer system by reducing the physical storage capacity in the storage apparatus which is to be pre-prepared.
Furthermore, as a data management method for this kind of storage apparatus in which the thin provisioning function is installed, a hierarchical data management method has conventionally been proposed (PTL 1, for example). A hierarchical data management method is a method in which storage areas provided by each of a plurality of types of storage devices of varying performance which are built into the storage apparatus are managed as storage tiers of a plurality of different types, where storage areas are assigned from a high-speed, high-performance storage tier to areas where data of high access frequency in the virtual volume is stored, and storage areas are assigned from a low-speed, low-performance storage tier to areas where data of low access frequency in the virtual volume is stored. With this hierarchical data management method, the cost performance of the storage apparatus can be improved.